If weather and technical issues don't scrub Monday's attempt, Endeavour will make its 25th and final missioin. The youngest of the shuttle fleet's orbiters is slated to spend 16 days in space and upon returning home be prepped for its retirement to the California Science Center in Los Angeles.

Endeavour's main payload is the Alpha Magnetic Spectrometer, a particle physics experiment device that will be mounted to the ISS. Endeavour is also bringing the final of four Express Logistics Carriers also to be mounted to the ISS as well as spare parts including two S-band communications antennas, a high-pressure gas tank, spare parts for Dextre and micrometeoroid debris shields.

The first launch attempt was scrubbed on April 29 after a heater for one of the shuttle's three Auxiliary Power Units failed. The APUs provide hydraulic power to steer the shuttle on both launch and landing. It was determined a circuit in a switchbox that directed power to the heater had shorted out.

Despite not being able to pinpoint the reason for the short, mission managers earlier this week said they had replaced enough parts and tested enough on board the shuttle to make the decision to try for the Monday launch.

Space Shuttle Launch Integration Manager Mike Moses said NASA technicians installed and tested new wiring that bypasses the original electrical wiring that connects the switchbox, which was also replaced, to the heaters. NASA did not replace any of the heaters.

"We now have extremely high confidence the problem is no longer on the ship or in any of the electronics," Moses said. "We have nothing in front of us preventing us from launching on the 16th."

Shuttle Launch Director Mike Leinbach later added that if the problem does come up again on launch day, it's still a scrub situation. All the heaters on the shuttle must work in order for it to launch.

The delay in the STS-134 launch will also mean the planned launch of space shuttle Atlantis on STS-135, the final mission of the shuttle program, will slip from its original planned date of June 28 to at least the second week of July, and Moses ruled out the iconic launch date of July 4.